The present invention relates to a distance-measuring device for measuring a distance to an object by using a laser beam.
In recent years, a non-prism distance-measuring device has been widely propagated. This device projects a laser beam for distance measurement directly to an object to be measured and measures a distance to the object to be measured.
In the non-prism distance-measuring device, a laser beam with a smaller beam diameter is used. By the use of the laser beam with the smaller beam diameter, the laser beam can be pinpointed to the object to be measured. As a result, a measuring position of the object can be accurately identified, and a ridgeline or a specific point on the object can be measured.
The intensity of the projected laser beam is limited from the reasons such as safety. In this respect, in a non-prism distance-measuring device, in which it is not possible to have high reflection from the object to be measured, the measuring distance is shorter compared with a distance-measuring device using a prism (corner cube).
For this reason, a prism is used as the object to be measured in long-distance measurement. To facilitate collimation and to attain highly accurate measurement, a laser beam with relatively large beam spreading is used.
As described above, the beam diameter of the laser beam is smaller in the non-prism distance-measuring device, and it is difficult to project the laser beam to the prism. Thus, it is not adequate to use the prism for long-distance measurement.
However, it is not very economical to install both a distance-measuring device for long distance using a prism and a non-prism distance-measuring device. In this respect, a distance-measuring device is now proposed, by which the distance measurement using a prism and the non-prism distance measurement can be achieved by a single distance-measuring device.
For instance, as described in JP-A-2000-88566 (FIG. 1; Paragraphs [0029]–[0035]), a distance-measuring device has been proposed, by which distance measurement using prism and non-prism distance measurement can be performed by a single distance-measuring device.
Referring to FIG. 7, brief description is given.
There are provided a first light source 2 for emitting a visible laser beam 1 and a second light source 4 for emitting an infrared laser beam 3, and the visible laser beam 1 and the infrared laser beam 3 can be emitted separately. The visible laser beam 1 is a laser beam with a smaller beam diameter and having parallel luminous fluxes. The infrared laser beam 3 is a divergent laser beam.
The visible laser beam 1 and the infrared laser beam 3 are selected according to a type of an object to be measured. For instance, when the object to be measured 5 is a reflective object such as a corner cube, the divergent infrared laser beam 3 is projected. When the object to be measured 5 is a wall surface of a building, etc., for instance, the visible laser beam 1 with a smaller beam diameter is projected. A reflection light 11 from the object to be measured 5 is received by a detector 8 through an objective lens 6 and a filter 7. Based on a signal from the detector 8, a distance to the object to be measured 5 can be calculated by an arithmetic unit 12.
The filter 7 transmits only light components in wavelength range of the visible laser beam 1 and the infrared laser beam 3. Unnecessary light components such as solar light are cut off. This contributes to the improvement of detection accuracy to detect the reflection light 11 of the detector 8.
In the conventional type distance-measuring device as described above, two light sources are used, and this means that the complicated design is required for the light emitting unit in the features such as control of the light sources. Because the visible laser beam 1 and the infrared laser beam 3 are used, the filter 7 is designed to fit the wavelength ranges of both of the laser beams. FIG. 8 shows the relation between wavelength transmission characteristics A of the filter 7 and photodetection characteristics B of the detector 8 and also the relation between wavelengths of the visible laser beam 1 and the infrared laser beam 3.
The filter 7 transmits the light components with wavelengths of the visible laser beam 1 and the infrared laser beam 3, and the filter 7 has such characteristics as to transmit light components having a wavelength longer than a transmission wavelength of the visible laser beam 1. Therefore, when a light beam of a wavelength range wider than that of the visible laser beam 1 and the infrared laser beam 3 enter and the detector 8 receives the reflection light 11, an S/N ratio of the reflection light 11 with respect to disturbance light is smaller. An avalanche photodiode (APD) which is generally used as the detector 8 has such photodetection characteristics that sensitivity is at the highest in the wavelength range of about 780 nm. In particular, when measurement is performed with the visible laser beam 1 as in the conventional type device, the reflection light 11 is detected when the sensitivity of the detector 8 is low. In most cases, disturbance light is solar light. Wavelength distribution of solar light is as shown in FIG. 9, and the wavelength distribution is widely spread over the range beyond the visible range. Therefore, when the filter 7 as described above is used, which has such characteristics as to transmit the light components with a wavelength longer than the wavelength of the visible laser beam 1, there is a problem in that the S/N ratio is still lower.